In the related art, a technique is proposed such as a MIMO (Multiple-Input Multiple-Output) communication system where data transmission rate is increased by transmitting different modulated signals at the same time from a plurality of antennas on a transmitting apparatus side and then demultiplexing the modulated signals mixed on the channel on the receiving apparatus side.
A configuration example of this type of communication system is shown in FIG. 1. Multi-antenna transmitting apparatus 20 inputs digital transmission signals 1A to 1D for channels A to D to modulated signal generating sections 2A to 2D. Modulated signal generating sections 1A to 1D then form modulated signals 3A to 3D for channels A to D by performing modulation processing such as BPSK (Binary Phase Shift Keying), QPSK (Quadrature Phase Shift Keying) and 16QAM (Quadrature Amplitude Modulation) on transmission signals 1A to 1D and transmits the signals to radio sections 4A to 4D. Radio sections 4A to 4D form transmission signals 5A to 5D for channels A to D of radio bands by performing predetermined radio processing such as frequency conversion on baseband modulated signals 3A to 3D and outputs transmission signals 5A to 5D to antennas 6A to 6D.
Multi-antenna receiving apparatus 30 receives the signals mixed on the channel after transmitted by the plurality of antennas 6A to 6D of multi-antenna transmitting apparatus 20. Multi-antenna receiving apparatus 30 receives signals mixed on the channel using antennas 7_1 to 7_4. Received signals 8_1 to 8_4 received by antennas 7_1 to 7_4 are inputted to radio sections 9_1 to 9_4. Radio sections 9_1 to 9_4 then obtain baseband signals 10_1 to 10_4 from received signals 8_1 to 8_4 for the radio band by performing predetermined radio processing such as frequency conversion on received signals 8_1 to 8_4 and output baseband signals 10_1 to 10_4 to demultiplexing/demodulating section 11. Demultiplexing/demodulating section 11 obtains a spatial correlation matrix between transmit antennas 6A to 6D and receive antennas 7_1 to 7_4 based on, for example, a known preamble inserted within modulated signals 3A to 3D, demultiplexes and extracts signals corresponding to modulated signals 3A to 3D using an inverse of this matrix, and demodulates demultiplexed and extracted the signals corresponding to modulated signals 3A to 3D so as to obtain digital received signals 12A to 12D corresponding to digital transmission signals 1A to 1D.
Further, in the related art, many techniques have been proposed to increase a data transmission rate in a multi-antenna communication system. For example, in non-patent document 1, a technique is proposed for increasing a data transmission rate by appropriately switching the number of transmission signals and the modulation schemes at modulated signal generating sections 2A to 2D.    Non-patent document 1: “A Throughput Enhancement for MIMO-OFDM Systems using Transmission Channel Control and Adaptive Modulation” Institute of Electronics, Information and Communication Engineers, technical report RCS-2003-263, January 2004.